In vivo optical imaging of internal organs of a patient is commonly performed through a fiber-optic catheter. Many clinical areas such as cardiology, interventional radiology and gastroenterology require a small diameter, rotating optical probe or catheter to generate r-□ cross-sectional images. In addition, the rotating catheter may be pulled back along a longitudinal direction to obtain three dimensional images of the tissue volume of interest. For this application, a catheter providing a focused optical beam and connectivity to the imaging system may be an important device. The optical imaging system can include optical frequency domain imaging and optical coherence tomography.
Generally, ideal characteristics of fiber-optic catheters may include: a) a narrow diameter, b) a high flexibility, and c) a low optical aberration. Since an optical fiber can easily be produced with a diameter less that 250 μm, fiber-optic probes have the potential for minimally invasive access to small vessels and narrow spaces within living subjects. Typically, catheters are directed to locations of interest through the use of a guide-wire that is placed under fluoroscopic guidance. To achieve compatibility with the guide-wire, and additionally to protect the optical fiber, catheters typically utilize an outer transparent sheath. The optical fiber can be placed inside of the sheath and is free to rotate or translate longitudinally. Light transmitted through the fiber is directed to a path perpendicular to the longitudinal axis of the catheter and focused to a point outside of the sheath, within the tissue of interest. As the light propagates through the sheath, its focal properties are modified by refraction at the inner and outer surface of the sheath. In other words, the sheath acts as a lens. Due to the cylindrical shape of the sheath, however, its lens characteristics may be undesirable and, in particular, can introduce significant aberrations. One of the most significant aberrations of the sheath is astigmatism, an effect that increases dramatically when using narrow diameter sheaths. Light rays passing through an optical element having astigmatism would exhibit two distinct foci, one focus for rays in the sagittal plane and another focus for rays in the orthogonal, tangential plane. An arrangement (e.g., a catheter) that overcomes this limitation would improve optical imaging, and may have widespread applications in medicine and biology, in particular.
One approach to overcome astigmatism introduced by the sheath can be to match the index of refraction of the sheath with the medium outside of an inside of the sheath. For biological imaging, this can be approximated by using a sheath having an index of refraction approximately equal to that of water, and to fill the lumen of the sheath with water or a substance of approximately equal index of refraction. It is highly desirable for the optical imaging catheter to enable both rotation and longitudinal pull-back of the components internal to the sheath. Although a rotation of the internal components within a water-filled sheath is possible, a longitudinal pull-back is problematic due to the viscosity of the fluid and turbulence. A more desirable solution may be to compensate the astigmatism of the sheath using other optical components, and to operate the catheter with air or another gas occupying the void between the internal components and the sheath.
It is known in the art that miniature lenses, having diameters approximately equal to that of standard optical communications fibers, can be used to shape the light emitted from an optical fiber to form a focal spot external to the fiber. It is also well-known that these devices can collect light from a focal spot and transmit that light backward through the optical fiber.
FIGS. 1a-1d show exemplary conventional configurations for combining miniature lenses and optical fiber. For example, in order to achieve a small package size, approximately equal to the diameter of optical fibers (less than approximately 500 μm), a gradient-index (GRIN or SELFOC) lens 25 is typically used. Commonly, the protective outer layer 10 of a glass optical fiber is partially stripped back from an end of the fiber 15, and a lens 25 is fixed to the fiber using optical adhesive or optical epoxy. In the case of a gradient-index lens, light emitted from the core 20 of the fiber follows a path whose marginal rays 30 describe a sinusoid. Through an appropriate selection of the index-of-refraction profile in the material of the lens and the lens length, the focal properties of the light emitted from the lens can be controlled. A common configuration for such a lens-fiber combination provides a focal spot 35 at a predetermined distance from the distal face of the lens. In addition to a lens, a beam deflector such as a prism 90 can be used to redirect the light 85 emitted from the lens to illuminate a focus 80 located transversely with respect to the axis of the fiber. In order to minimize a back-reflection from the lens and to improve the mechanical integrity of the device, the lens may be directly bonded or fusion spliced to the optical fiber. Alternatively, a spacer 105 that includes a glass cylinder of homogeneous index of refraction can be inserted between the fiber 100 and the lens 115 to allow for beam expansion 110 prior to focusing. A prism or beam deflector 120 can further be used to redirect the beam to a focal spot 125 located at a position with a transverse offset with respect to the axis of the fiber.
For each of the probes illustrated in FIGS. 1a-1c, the length of the lens and spacer must be carefully controlled and the elements carefully aligned to achieve the desired focal characteristics for a specific application. As a result, such probes are difficult to manufacture. Additionally, these designs lack mechanical integrity and require an additional structure, such as an outer protective sleeve, to avoid mechanical failure. This requirement may result in a larger probe diameter and longer rigid length than otherwise might be possible.
Ball lenses that include a single spherical particle of glass can alternatively be used to produce a focus from light emitted from an optical fiber. In this case, as shown in FIG. 1d, the light 130 emitted from the fiber is refracted at the surface of the sphere 135. The ball lens can be positioned at the distal end of the fiber or can be formed directly from the material of the fiber by controlled heating and melting of the glass. During the heating process, a portion of the light-guiding core of the fiber 125 can be destroyed and the light can diffract to a larger beam size at the ball-lens external surface 135 producing improved focal characteristics 140. An important aspect of the device shown in FIG. 1 is that the ball lens is fabricated by melting and reforming the distal end of an optical fiber is that the surface of the ball is approximately spherical over the portion where light is transmitted. Additionally, a beam deflector such as a prism cannot be directly bonded to the spherical surface of the ball lens, thus requiring an additional housing for its positioning and mechanical fixture.
Therefore, there is a need to overcome at least some of the deficiencies described herein above.